Molecular analysis of psbA mutations responsible for various herbicide resistance phenotypes in Synechocystis 6714

Mutations conferring herbicide resistance in 3 mutant strains of the cyanobacterium Synechocystis 6714 have been characterized by gene cloning and sequencing. The mutants display very different phenotypes: DCMU-IIA is DCMU-resistant and atrazine-resistant, DCMU-IIB is DCMU-resistant and atrazine-sensitive, and Az-V is DCMU-sensitive, atrazine-resistant and presents particular photoinhibition properties. These mutants were originally obtained either by one-step selection (DCMU-IIA) or by two-step selection (DCMU-IIB and Az-V). psbA copies carrying herbicide resistance have been identified by transformation experiments as psb AI in all cases. Sequences of the psb AI copy of each mutant have been compared to the wild-type sequence. In the single mutant DCMU-IIA, a point mutation at codon 264 (Ser----Ala) results in resistance to both DCMU and atrazine. In the double mutants DCMU-IIB and Az-V, two point mutations were found. DCMU-IIB was derived from DCMU-IIA and had acquired a second mutation at codon 255 (Phe----Leu) resulting in a slight increase in DCMU resistance and complete abolition of atrazine resistance. Az-V contains two changes at codons 211 (Phe----Ser) and 251 (Ala----Val) resulting in high atrazine resistance but only slight DCMU resistance.